1. Field of the Invention
The present invention relates to a display apparatus using a reflective display panel.
2. Description of the Prior Art
Recently, attention has been given to a reflective liquid crystal display (LCD) having higher display efficiency than a transmissive LCD. The reflective LCD acting as a reflective display panel reflects illumination light incident on the surface thereof at a reflection angle substantially the same as the incidence angle but opposite thereto in sign so that the light exits therefrom as so-called projection light of regular reflection. Heretofore proposed display apparatuses using the reflective LCD include one in which the reflective LCD is illuminated from a direction substantially vertical to the surface thereof so that the projection light exits in the vertical direction and images are formed by passing the projection light through a projection optical system.
The structure of the conventional display apparatus is shown in FIG. 29. In the figure, reference numeral 101 represents a light source, reference numeral 102 represents a reflector coupled to the light source 101 and reflecting light from the light source 101 to condense it, reference numeral 103 represents an illumination optical system disposed ahead of the reflector 102 for uniformly illuminating a subsequently-described reflective LCD with efficiency, reference numeral 104 represents a polarization beam splitter, reference numeral 105 represents the reflective LCD, reference numeral 106 represents a projection optical system, and reference numeral 107 represents a screen.
As shown in the figure, illumination light emanating from the light source 101 and the reflector 102 passes through the illumination optical system 103 to be incident on the polarization beam splitter 104. Then, only an s-polarized luminous flux is reflected at the polarization beam splitter 104 to be vertically incident on the surface of the reflective LCD 105. The vertically incident illumination light is selectively converted into p-polarized light pixel by pixel by the reflective LCD 105 and is totally reflected. Of the projection light vertically totally reflected at the reflective LCD 105, only p-polarized light passes through the polarization beam splitter 104 to be imaged on the screen 107 by the projection optical system 106.
Another reflective display panel receiving attention is a digital micromirror device (DMD). The surface of the DMD is divided into a plurality of pixels each having a fine mirror (micromirror), for example, being square. The micromirror rotates about two opposite angles of the pixel so as to incline +/-10 degrees pixel by pixel. For example, the condition where the micromirror inclines +10 degrees is set as the ON condition and the condition in which the micromirror inclines -10 degrees is set as the OFF condition.
FIG. 30 is a perspective view showing a reflection image of a micromirror of the DMD. In the figure, reference numeral 111 represents a DMD acting as the reflective display panel, reference numeral 112a shown by the solid line represents the micromirror at a pixel of the DMD 111 which micromirror is in the ON condition, reference numeral 112b shown by the broken line represents the micromirror which is in the OFF condition, and reference numeral 113 represents a projection optical system disposed above the DMD 111 and forming images by transmitting subsequently-described projection light from the DMD 111.
As shown in the figure, the rotation axis ab of the micromirror 112a forms, as shown by the arrow e, an angle of 45 degrees to the short sides c or the long sides d of the rectangle constituted by the DMD 111. The display apparatus using the DMD 111 constitutes a non-illustrated illumination optical apparatus so that illumination light A51 is incident at an incidence angle of 20 degrees to the surface of the DMD 111 within a cross section vertical to the rotation axis ab, that is, within a surface forming another angle of 45 degrees to the short sides c or the long sides d as shown by the arrow f.
When reflected at the micromirror 112a being in the ON condition, the illumination light A51 becomes projection light B51 with a reflection angle of 0 degree to the surface of the DMD 111. When reflected at the micromirror 112b being in the OFF condition, the illumination light A51 becomes projection light B52 with a reflection angle of -40 degrees to the surface of the DMD 111. The projection optical system 113 forms images by use of only the projection light B51 which is a luminous flux with a reflection angle of 0 degree.
The sum of the incidence and reflection angles will be referred to as incidence-reflection angle characteristic and is defined as "incidence angle+reflection angle" with respect to the incidence and reflection angles expressed as angles with signs like the above. For panels exhibiting the regular reflection characteristic like the above-described reflective LCD, the "incidence angle+reflection angle"=0 degree. For pixels where the micromirror is in the ON condition in the above-described DMD, the "incidence angle+reflection angle"=20 degrees.
However, since the reflective LCD has the regular reflection characteristic that vertically incident illumination light is vertically reflected, in the arrangement as shown in FIG. 29, the illumination light and the projection light take substantially the same optical path in opposite directions, so that an element for separating the optical path of the illumination light and the optical path of the projection light, for example, the above-mentioned polarization beam splitter is necessarily provided. This requires a large glass block and the employment of multi-layer thin film processing. As a result, the cost increases.
The polarization beam splitter selects the luminous fluxes to be passed therethrough based on the direction of the polarization plane of the reflected light (projection light) from the reflective LCD. However, there are occasions when, because of disturbance of the polarization plane caused by nonuniformness in homogeneity in the glass block, unnecessary ray components are passed through the beam splitter to decrease the contrast. In order to facilitate the separation between the optical path of the illumination light and the optical path of the projection light, the reflective LCD is illuminated from a direction oblique to the surface thereof. However, in that case, the projection light exits in the opposite oblique direction because of the regular reflection characteristic, and it is extremely difficult to structure a projection optical system that forms excellent images being conjugate with the reflective LCD based on the projection light exiting in the oblique direction.
On the contrary, in the DMD, since reflection light vertical to the panel is obtained by causing illumination light with an incidence angle of 20 degrees to be incident on the surface of the DMD within a surface forming an angle of 45 degrees to the short or the long sides of the panel, the optical path of the illumination light and the optical path of the projection light can be separated, so that the problem that arises when the above-described reflective LCD is used is solved. However, the surface forming an angle of 45 degrees to the short or the long sides is substantially in a diagonal direction with respect to the panel of the DMD, and in the surface, the image height is largest viewed from the projection optical system. It is necessary to secure the lens back focal length (LB) of the projection optical system in accordance with the image height. Since the reflected light from the panel is in a vertical direction, it is necessary to fulfill predetermined conditions such that the projection optical system should be in telecentric condition. For these reasons, display apparatuses using the DMD are large in size.